Mouse strains have proven to be important for use as model systems in studies of various human diseases such as types of malignancies. Over the last 50 years or so researchers have used both inbred and outbred mice to study both spontaneous and induced neoplasms. Murine leukemia viruses (MuLVs) have been implicated in the spontaneous development of hematopoietic neoplasms involving both T and B cells in mice. Inbred mouse strains vary markedly in their incidence of spontaneous leukemia. Mice of high leukemic strains (e.g. AKR, C58 etc.) contain large amounts of ecotropic MuLV early in life. Low leukemic strains fall into two categories virologically. Some strains demonstrate ecotropic virus, but less often and later in life (e.g. BALB/c, C57BL etc.); other strains never yield ecotropic virus (e.g. NIH Swiss, 129, C57L etc.). High virus, high leukemic strains, e.g. AKR, C58 predominantly develop T-cell lymphoma and were extensively studied. These strains contain 3 or more loci for ecotropic virus in their genomes. With the advent of recombinant DNA technology various genetically altered mice have been developed. Most of these strains were developed on a background of C57BL/10 (B10), C57BL/6 (B6), 129 mice. Spontaneous disease incidence in these mice is not well characterized. Recently we analyzed 25 lymphomas from aging B6 mice. All of these mice developed only B cell lymphomas as determined by immunoglobulin heavy chain gene rearrangement study and none of them had any T-cell receptor beta chain arrangements indicating that there is no T-cell involvement in these tumors. On the other hand, aging B6-Swiss mice developed a mixture of T cell and B cell lymphomas. B6 mice harbor only one copy of the N-tropic ecotropic MuLV genome. These genomes occasionally yield infectious virus, but then in old age. In order for the spontaneously activated virus to spread, the p30 region of the gag gene requires change either by mutation or by recombination to produce B-tropic virus, which can infect host cells. We found novel integration of ecotropic viral genome in only 2-3 tumor DNAs in 25 specimens, which were tested. The B6-Swiss tumor DNAs that we tested had no ecotropic genome to begin with. Thus in the development of lymphomas in B6 and B6 Swiss mice either another group of retroviruses e.g. MCF virus or factors other than retrovirally induced insertional mutagenesis are involved. We have been in a unique position to study various cellular genes involved in the development of mouse B cell lymphomas due to the availability of tumors and tumor-derived cell lines from NFS.V+ mice and other strains of mice expressing ecotropic virus at high levels. NFS mice do not normally express ecotropic virus due to the lack of the viral genome in their DNA. However, high virus expression was achieved in NFS mice infected with ecotropic virus or by developing congenic strains (NFS V+ congenic) using ecotropic viral gene from AKR or C 58. Insertional mutagenesis by proviral integration proved to be quite useful for identifying involved cellular genes. Using Southern analysis to compare each tumor DNA with its respective tail DNA, we determined if there were new ecotropic proviral integrations for each of these tumors. In order to enumerate all new integrations or to rule out concealed integrations, multiple restriction endonuclease digestions were sometimes necessary. About 90% of the total tumor DNAs had new somatic integration of ecotropic MuLV. We observed that not all of the novel integrations in the tumors appeared in the cell lines derived from them; on the other hand cell lines sometimes harbored novel integrations not obvious in the primary tumor. To begin with we chose two tumors and their corresponding cell lines to molecularly clone both 3' and 5' cellular sequences adjacent to the viral genome using bacteriophage lambda cloning system and the ecotropic virus-specific probe sequence. In one instance, the primary tumor had one novel viral integration whereas the cell line had a unique integration site, but one which was different from the tumor. We molecularly cloned and characterized both novel sites and the flanking cellular DNA was sequenced. In the second case there were six novel integration sites in the primary tumor and eight in the cell line of which four were in common with the primary tumor. We molecularly cloned 5 sites from the cell line. Sequencing of 10 host viral junction fragments and repeated blast searches revealed that these are hitherto unidentified sequences. Unique sequence probes were derived from the flanking cellular DNA in one of these clones. Screening of over 650 tumor DNAs revealed one other rearrangement involving the same cellular sequence. This DNA sequence maps in the middle of mouse chromosome 5. In order to identify the cellular gene(s) which may be involved, we are analyzing BAC clones containing about 130 kb of cellular sequences using this probe. We will continue to develop unique sequence probes from the molecularly cloned integration sites to determine their frequency of involvement as well as to determine which genes are involved.